The family of the cellulose ethers, used for a long time in many industrial fields, include water-soluble hydroxyalkyl celluloses (HAlkC) and related ethers, such as hydroxyethyl cellulose (HEC), hydroxypropyl cellulose (HPC), hydroxyethyl-hydroxypropyl cellulose (HEHPC), hydroxyethyl-methyl cellulose (HEMC), and hydroxyethylcarboxymethyl cellulose (HECMC). The properties, manufacturing processes and fields of application of these cellulose ethers are described, for example, in Ullmanns Encyklopaedie der technischen Chemie (Ullmanns Encyclopedia of Industrial Chemistry), Verlag Chemie--Weinheim, 4th Edition, Volume 9, 1975, keyword "Celluloseather" ("cellulose ethers"), pages 192 to 212.
A continuous process for the manufacture of HEC is known (Ullmann, page 206) in which 2 to 8% of ethylene oxide is absorbed, but this leads to water-insoluble products. Higher degrees of etherification are said to lead to a product having an undesirably pasty consistency, resulting in difficulties in the subsequent use of such product in manufacturing processes. Therefore, prior art processes for the manufacture of water-soluble hydroxyalkyl cellulose and mixed ethers thereof are normally carried out discontinuously as suspension ("slurry") processes in the presence of inert organic solvents, such as isopropanol, tert.-butanol or acetone.
British Pat. No. 482,885 describes a continuous process for the manufacture of water-insoluble, alkali-soluble cellulose ethers, such as MC, CMC or HEC, having a low degree of substitution, wherein a dissolved etherifying agent is reacted in a quantity of 0.1 to 2 moles under alkaline conditions with alkali cellulose, without applying pressure.
The continuous process for the manufacture of water-insoluble, alkali-soluble hydroxyethyl cellulose according to British Pat. No. 782,842, German Auslegeschrift No. 1,251,729 or U.S. Pat. No. 2,847,411 is carried out with gaseous ethylene oxide as the etherifying reagent in such a way that 2 to 8% by weight of ethylene oxide, relative to the weight of the cellulose, are reacted. A similar procedure using a proportion of ethylene oxide of less than 2% by weight is described in British Pat. No. 817,809.
German Pat. No. 765,441 has disclosed a continuous process for the etherification of cellulose, wherein alkali cellulose is reacted in single-screw or multi-screw devices with liquid organic etherifying agents; the only example of an etherifying agent mentioned is benzyl chloride.
U.S. Pat. No. 3,131,177 describes a discontinuous process for the manufacture of water-soluble hydroxyalkyl cellulose (HAlkC), in which cellulose slurried in a water-soluble ketone, such as acetone, is reacted with an alkylene oxide in the presence of alkali and water. The two-stage etherification process is carried out in such a way that: (a) the etherification is initially carried out at about 80.degree. C. with a quantitative ratio of 0.2 to 0.8 parts by weight of NaOH and 0.8 to 4.0 parts by weight of water per part by weight of cellulose to give a molar degree of substitution (MS) of 1.0; (b) 70 to 98% of the alkali is neutralized; and (c) the etherification is continued up to a MS of 1.5 to 3.0. A similar discontinuous, two-stage etherification procedure is also described in U.S. Pat. No. 3,131,176.
A discontinuous process for the manufacture of hydroxyethyl cellulose (HEC) is disclosed in German Auslegeschrift No. 1,518,979, which corresponds to British Pat. No. 1,038,757. The process is carried out with a mixture of isopropanol and tert.-butanol as a dispersing agent (slurrying agent). The proportion of isopropanol in the mixture is between 2 and 25% by weight. The parameters of this process are selected such that: (a) a mixture of dispersing agent, water, NaOH and cellulose is produced at 30.degree. C. in the course of 30 minutes; (b) ethylene oxide is added to this mixture in a reactor; (c) the temperature is adjusted up to 50.degree. C. over a period of 30 minutes and this temperature is maintained for 90 minutes; (d) the temperature is raised to 80.degree. C. and this temperature is maintained for 30 minutes; and (e) the reaction vessel is cooled, the NaOH is neutralized, the crude product is purified with 80% strength aqueous acetone followed by anhydrous acetone, and the purified product is dried at less than 90.degree. C. The quantitative proportions (in parts by weight) of the components are: 1 part by weight of dry cellulose; 2 to 50 parts of dispersing agent; 0.05 to 10 parts of NaOH and 1.2 to 2.5 parts of water. Ethylene oxide, 2,3-epoxypropan-1-ol, epichlorohydrin, butadiene monoxide and ethylene chlorohydrin are mentioned as hydroxyethylating agents. The process is said to be suitable both for the manufacture of water-soluble HEC having a MS of 1.5 to 2.5 and for the manufacture of water-soluble HEC having a MS of less than 1.0.
U.S. Pat. No. 3,296,247 discloses a discontinuous process for the manufacture of hydroxyethylhydroxypropyl cellulose (HEHPC) having a MS.sub.HE of 0.05 to 1.0 and a MS.sub.HP of 2 to 10. This product is soluble in cold water and polar organic solvents, insoluble in hot water, and is a thermoplastic. The etherifying agents ethylene oxide and propylene oxide are reacted simultaneously at about 70.degree. C. in a closed pressure vessel.
Continuous processes for the manufacture of certain other water-soluble cellulose ethers are also known in the art, in particular, processes for the manufacture of carboxymethyl cellulose (CMC), methyl cellulose (MC) and methyl-hydroxyalkyl cellulose (MHAlkC). U.S. Pat. No. 3,544,556 describes a continuous process for the manufacture of methyl cellulose (MC) or methylhydroxyalkyl cellulose (MHAlkC), in which alkali cellulose is reacted with excess liquid methyl chloride or a mixture of liquid methyl chloride and liquid alkylene oxide in the presence of likewise liquid dimethyl ether at an elevated temperature. The weight employed of the etherifying liquid components is 5 to 10 times that of the dry cellulose contained in the alkali cellulose and, additionally, 2 to 30% by weight of dimethyl ether, relative to methyl chloride employed, is used, the dimethyl ether being formed only by side reactions from the methyl chloride. The process comprises the following steps: (a) the mixture of alkali cellulose, etherifying agent and dimethyl ether is brought to 70.degree. C. to 95.degree. C. immediately after entering a reaction tube; (b) the reaction mixture is passed continuously in the same direction through the tube which is provided with a conveyor screw and with a heating jacket. The liquid exchange medium of the jacket flows in counter-current to the reaction mixture; (c) the methyl chloride vapor generated in the tube is condensed in order to maintain the reaction mixture at the requisite temperature; (d) the reaction mixture is continuously withdrawn; (e) the withdrawn reaction mixture is, with let-down, washed with water of 75.degree. to 95.degree. C. in co-current, the components escaping as gases being recondensed and recycled. The amounts of etherifying agents within said recycled components which were consumed during steps a to c are replenished, so that a constant content of dimethyl ether within said range is established in the reaction mixture flowing through the reaction tube; and (f) the MC or MHAlkC is isolated in a customary manner.
U.S. Pat. No. 4,017,671 discloses a process for the continuous manufacture of the sodium salt of carboxymethyl cellulose (NACMC), in which an etherification mixture, which initially contains alkali cellulose, formed from finely comminuted pulp and sodium hydroxide solution, sodium monochloroacetate and water, is conveyed through a reaction chamber at a temperature of at least 60.degree. C., which is kept constant in each section of the reaction chamber, in such a way that the residence time of the etherification mixture in the reaction chamber is adequate for reacting the sodium monochloroacetate initially contained therein. The process has the following features: (a) the etherification mixture conveyed through the reaction chamber contains 0.2 to 0.6 parts by weight of NaOH, 0.5 to 1.75 parts by weight of sodium monochloroacetate, 7 to 22 parts by weight of isopropyl alcohol and 1.4 to 11 parts by weight of water per 1 part by weight of pulp; (b) the temperature of the etherification mixture is maintained in the reaction chamber in the range from 60.degree. C. to 90.degree. C.; (c) the shearing forces are caused, by means of a homogenizer, to act on the etherification mixture before the latter enters the reaction chamber and at a temperature which is below 30.degree. C.; (d) the residence time of the etherification mixture in the homogenizing zone is less than 1/10 of its residence time in the reaction chamber; and (e) after the etherification has ended, the liquid constituents of the etherification mixture are separated by mechanical means from its solid constituents.
The process for the manufacture of polysaccharide ethers according to U.S. Pat. No. 4,015,067 is carried out in such a way that: (a) a slurry of a finely divided polysaccharide, an aqueous metal hydroxide and at least one etherifying agent is prepared in the absence of free oxygen, and the temperature is controlled such that the reaction of the alkali polysaccharide with the etherifying agent is kept under control; (b) the slurry is introduced substantially continuously into a tubular reactor which is free from obstacles which could impede the flow of the slurry; (c) the temperature of the slurry is adjusted such that a controlled reaction of the etherifying agent or agents with the alkali polysaccharide takes place, and the desired substitution of the alkali polysaccharide by the etherifying agent is achieved; and (d) the reaction is continued for a period of 10 to 180 minutes while the slurry is passed through the reactor. Cellulose is mentioned as a typical polysaccharide, and it is said that a reaction is also possible with alkylene oxides as the etherifying agent in a quantity of 2 to 20 parts by weight (in particular 6 to 12 parts by weight) relative to 1 part by weight of cellulose.
The processes known from the state of the art, however, have in particular the following disadvantages:
The continuous processes for the manufacture of HEC only lead to a water-insoluble, alkali-soluble HEC, that is to say, under the process conditions used therein, only a low degree of etherification is possible, insufficient to render the HEC thus formed water-soluble.
The discontinuous processes for the manufacture of water-soluble HAlkC, in particular HEC, admittedly give products which can be used industrially, but they cannot be converted without difficulty to a continuous procedure, since a uniform course of the reaction, which is necessary for good solution properties of the products formed, would then no longer be guaranteed.
The continuous processes for the manufacture of the water-soluble cellulose ethers CMC, MC or MHAlkC do not, without further measures, also permit a manufacture of HAlkC or mixed ethers based on HAlkC, since they are carried out, for example, without applying pressure (CMC) and hence make different demands on the type of process and the design of the unit, or since the mixture of etherifying agent and alkali cellulose (MC, MHAlkC) can be reacted substantially without the presence of significant amounts of an inert organic solvent without decisive deterioration with respect to the quality demands, and this mixture is reacted after it has passed only through a relatively simple homogenization step. In particular, the two last-mentioned process parameters, namely carrying out the process in a slurry containing a solvent, and intensive and effective homogenizing before carrying out the actual etherification reaction, require special measures in process control and in the design of the unit.
Although it is mentioned in U.S. Pat. No. 4,015,067, already listed above, that a reaction of cellulose with epoxides, such as ethylene oxide, propylene oxide or butylene oxide, is also possible by the continuous process (col. 5, lines 14-19), concrete data are to be found in the further text only for running the process with monohalogenoalkanes, such as methyl chloride, ethyl chloride, propyl chloride or butyl chloride, to give the alkyl celluloses or especially with alkylene oxides in addition to give the corresponding mixed ethers. A fully continuous procedure is not envisioned, since (col. 2, lines 10-20) the forward motion of the slurry is occasionally interrupted in order to control the temperature in the course of the process, and moreover, the control of the process is also effected via the feed of the individual reactants into the tubular reactor (col. 1, lines 66-68). Regarding the quantity of etherifying agent and inert diluent, it is stated that these are to be used in a quantity of about to ten to twenty times the weight of the cellulose (col. 4, lines 5-6).